Carburetor Troubleshooting Guide.
Problem
Cold Starting
CONDITION
Engine cranks but will
not start.
POSSIBLE CAUSE
CORRECTION
1) Inspect choke adjustment and for
1) Choke not closing.
something binding. Adjust if necessary.
2) Lube with WD-40 and check for
2) Choke linkage binding. something bent. Adjust if necessary.
3) Check fuel delivery. Look for plugged
filter or clogged lines, bad pump, stuck
3) No gas in carb.
needle & seat, and fuel pressure.
4) Replace the pump. Problem is usually
caused by bad gas, dirt in gas, or vacuum
leak or ignition problems causing engine
4) Accelerator pump
spitback.
defective or blown out.
5) No spark or engine
problems such as bad
compression.
5) Diagnose & correct the problem.
Engine starts, then dies
within a few seconds.
1) Choke not closing
properly.
2) Big vacuum leak on
engine somewhere.
3) Choke pull off setting
incorrect.
4) Fast idle RPM set too
slow.
1) See notes above. Adjust choke if
necessary.
2) Use vacuum gauge to check. Fix the leak.
You may have put the base gasket on wrong
or it is the wrong one for this carb & engine
combination.
3) Adjust to factory specs. Carb can get
bumped in shipping or transportation and
can accidentally change the setting.
4) Adjust to recommended RPM.
5) Correct delivery to carb. Usually it is a
plugged up filter.
5)Low fuel delivery.
6) Electrical or
compression problems on 6) Do complete tune up & diagnosis. Fix the
the engine.
problem.
7) Check & adjust the float level to factory
7) Float level set very low. specs.
8) No voltage/ground to
8) Check for power & ground. Repair the
the idle solenoid.
problem.
9) Defective idle solenoid. Replace/repair problem.
Engine normally starts
OK but then dies backing
out the driveway or at the
first stop sign. After that it 1) Choke not set tight
runs OK.
enough.
1) Set choke a little tighter and retry.
2) The setting varies depending upon the
exact application. One carb may fit several
2) Choke pull-off opening applications. Set the pull-off so the choke is
too much.
a little tighter.
Engine starts OK,
increases RPM then gets
too slow with lots of black
smoke.
1) Choke set too tight.
1) Adjust a little (1/8") looser.
2) Pull-off set too tight.
2) Adjust so it is open a little more.
3) Slow flooding.
3) Fix cause of flooding.
4) Float level very high.
4) (Rare) Set to factory specs.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
5) Ford& Holley Only.
Power valve blown.
CORRECTION
5) Replace the power valve. Possibly
damaged by a backfire condition.
6) Pull-off diaphram
blown.
6) Caused by installer allowing engine to spitback up through carb. Replace the pull off.
7) Carter Only. Pump
blown out.
7) Caused by installer allowing engine to spitback up through carb. Replace the pump.
8) Crushed float.
8) Caused by installer allowing engine to spitback up through carb. Replace the float.
9) Choke spring may be
backwards and is getting
tight when heating up
instead of loose.
Engine starts, then races
for a few seconds and
1) Big vacuum leak
then dies every time.
somewhere.
Warm
Starting
Engine cranks but will
not start.
1) Carburetor flooding.
2) Choke is closed when
engine is hot.
1) Fix cause of flooding.
2) Find & fix cause for choke staying closed.
Look for no heat source, spring in
backwards, or something jammed or bent.
3) No spark.
3) Do complete tune up.
4) No compression.
6) No air.
4) Diagnose cause of no compression & fix.
5) Check fuel delivery volume and pressure.
Look for clogged lines, filter, or pump. Check
for kinked or swollen fuel lines.
6) Check for clogged air filter, especially after
driving through muddy or dusty area.
7) Too much air.
7) Look for big vacuum leak, such as broken
hose, blown gasket, bad power brake
diaphram, bad PCV valve (Rough Idle).
5) No fuel
Engine starts, then dies
within a few seconds.
9) Remove choke cover, cool off choke,
reverse spring, reinstall cover and set
tension to factory specs.
1) Correct the vacuum leak. Make sure you
haven't forgotten to hook up a hose
somewhere. Base gasket may be wrong one
or on wrong.
1) Choke is staying closed 1) Diagnose and fix choke problem.
2) Carburetor flooding.
3) Ford& Holley Only.
Power valve blown.
2) Fix cause of flooding.
3) Replace the power valve. Possibly
damaged by a backfire condition.
4) Check out entire fuel system venting
system, inluding the vent valve on the carb,
the charcoal canister, all hoses & check
valves in the system, and any solenoids that
4) Venting system failure. trigger the vent system to operate.
5) Idle jet plugged up with 5) Clean out the idle jet and any other dirt in
dirt.
the carb.
6) Idle air bleed plugged 6) Check idle air bleed. Clean or replace as
up or missing.
necessary.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
CORRECTION
7) Idle cut-off solenoid not 7) Check idle solenoid, especially for
working.
power/ground, replace solenoid if necessary.
1) Find the vacuum leak & fix it. Also look for
wrong base gasket or one that is installed
Engine starts, then races
wrong. If engine has been spitting back, it
for a few seconds and
1) Big vacuum leak
may have blown out the base gasket or a
then dies.
somewhere.
gasket in the carb.
Engine starts OK, but
then gets real rough.
Lots of black smoke.
1) Ford& Holley Only.
Power valve blown out by 1) Replace the power valve. Possibly
spitback up through carb. damaged by a backfire condition.
2) Slow flooding.
3) Crushed float.
4) Venting system
problem.
Cold Engine
Driveability
Condition.
Engine stalls when
transmission is put into
gear.
1) Incorrect choke pull-off
adjustment.
2) Fast idle RPM incorrect
(too slow)
3) Engine running too lean
because of vacuum leak
or dirty jet.
Hesitation, stalling,
stumbling, flatspot, or
deadspot during
acceleration: Backfiring
or spitback through carb. 1) Vacuum leak.
2) Ignition timing retarded
too far.
3) Accelerator pump
nozzle has dirt in it.
4) Accelerator pump cup
swollen up from contact
with bad gas or
chemicals.
5) Economizer jet too
small or partially blocked.
6) Choke pull-off open too
far.
7) Secondary throttle
plates not closing all the
way.
8) Vacuum hoses hooked
up wrong.
9) Idle jet partly blocked
with dirt.
2) Fix cause of flooding.
3) Caused by installer allowing engine to spitback up through carb. Replace the float.
4) Check entire venting system & repair.
1) Readjust the pull-off or replace if
necessary.
2) Speed up the fast idle to factory specs.
3) Check for vacuum leak. Flow test carb to
check jetting.
1) Check for vacuum leak & repair.
2) Reset timing.
3) Clean out the nozzle tip.
4)
Replace the pump cup.
5) Clean out economizer jet & check the
size.
6) Adjust the pull-off tighter.
7)Check for binding or bent linkage. Repair.
8) Connect up right. Be especially careful of
the EGR & Dist. connections: sometimes the
pipe locations are reversed on Rochesters.
9) Clean out the jet & any other dirt that is in
there.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
Hesitation, deadspot or
stalling after first
warmup.
POSSIBLE CAUSE
CORRECTION
10) Distributor timing not
advancing properly (worn
breaker plate, worn shaft,
pin hole in distributor
diaphragm, cracked hose, 10) Check distributor and all related systems
etc.)
carefully. Replace defective parts.
1) Defective electric assist
choke.
1) Replace it.
2) Defective accelerator
pump (low output).
2) Replace it.
3) Float level setting very
low.
3) Adjust to factory specs.
4) Bad ignition condensor. 4) Replace it.
Periodic backfiring with
black exhaust smoke:
Deadspot, flatspot,
hesitation, stumbling,
backfiring.
1) ) Inspect and clean passages in intake
1) Plugged heat crossover manifold and heads. Test heat riser valve,
system in manifold.
replace if defective.
2)
Check and replace as necessary: heat
2) Defective source of hot shroud duct, temperature sensor, vacuum
air up to the carburetor.
door motor, manifold vacuum supply.
Warm Engine Hesitation under light
Driveability throttle: Deadspot &
stumble.
Condition.
1) Vacuum leak
somewhere, or hose off or
hooked to wrong vacuum
port.
2) Accelerator pump
problems.
1) Inspect hoses. Route and lead the hoses
correctly. Look for leaks caused by the
wrong base gasket or if it was installed
upside down.
2) Inspect and adjust pump stroke, pump
plunder, discharge nozzles and check
valves. Inspect the accelerator pump, look
for swollen pump cup.
3) Float level set very low. 3) Set float to factory specs.
4) Set to factory specs. Make sure
4) Ignition timing retarded. advancing correctly.
5) Dirty idle jet or
economizer jet.
5) Inspect idle jets. Clean as necessary.
6) Idle speed set too fast 6) Richen up the idle mixture, reset the idle
& mixture is too lean
speed to factory specs., then reset the
(common)
mixture using the lean drop method.
Doggy, runs rough, lots
of black smoke at idle.
7) Idle cut-off solenoid not
working.
8) Frozen or binding
heated air inlet (stuck in
full hot or full cold
position).
9) EGR valve stuck on or
coming on too early (hose
on wrong?)
1) Choke staying closed
or partly closed.
7) Check idle cut-off solenoid, especially for
power/ground, replace solenoid if necessary.
2) Slow flooding.
2) Fix cause of flooding.
8) Inspect and repair, as necessary.
9)
Inspect hose routing to EGR valve & inspect
valve. Replace as necessary.
1) Fix choke or heat source.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
CORRECTION
3) Ford& Holley Only.
Power valve blown out by 3) Replace the power valve. Possibly
spitback up through carb. damaged by a backfire condition.
4) No power or heat
source to choke.
4) Repair as necessary.
Hesitation under heavy
throttle: Deadspot &
stumble. May backfire or Defective accelerator
pump.
spitback.
1)
1) Look for dirt in pump nozzles, swollen cup
from bad gas, or check ball missing or stuck.
2) Metering rods or power
valve sticking or binding. 2) Repair as necessary.
3) Vacuum leak.
4) Float level setting very
low.
5) Plugged up fuel filter,
defective fuel pump, or
swollen or kinked lines.
6) Secondary air valve set
wrong.
3) Locate leak and repair.
4) Reset to factory specs.
5) Inspect and repair as necessary.
6) Check & adjust the secondary air valve
spring.
7) Set to factory specs. Make sure
7) Ignition timing retarded. advancing correctly.
Dies coming up to a stop 1) Bad or misadjusted
BCDD (if equipped).
sign, but idles OK.
2) Bad throttle positioner
or bad vacuum source to
it.
1) Adjust to specs or replace as necessary.
2) Check throttle positioner with a vacuum
pump. Replace if defective. Replace
cracked hoses. Check vacuum hose
connected to the correct pipe on carburetor
or thermal switch.
3) Idle speed and mixture 3) Reset to specs. See the adjustment
incorrectly adjusted.
instructions.
4) Loose or defective float
pin.
4) Fix or replace the pin.
Poor
Performance
or Gas
No power or bad gas
mileage.
Mileage.
1) Plugged exhaust.
1) Check exhaust. Look for plugged catalytic
converter, bad muffler baffle, kinked or
crimped pipe, dirt or other foreign matter in
pipe.
2) Remove gas cap & see if performance
improves. If so clean or replace the gas cap.
Check the charcoal cannister, hoses to it &
any check valves. Check the electric vent
2) Clogged gas tank vent, valve on the carb if there is one, & make
or fuel venting system.
sure it is getting power at the right time.
3) Check timing at idle and also for full
advance when revved up. Look for bad or
loose hose, hose hooked up to wrong pipe
on distributor, bad thermal switch, leaking
vacuum advance can on the distributor, a
worn breaker plate, worn distributor shaft,
3) Ignition timing retarded sticky weights, point gap closed up, etc.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
4) Clogged air filter
CORRECTION
4) Replace air filter.
5) Choke not opening
5) Fix choke or heat source problem.
6) Check the lockout: secondaries won't
open unless choke is coming off all the way.
Check for sticky or bent shaft or linkage. If air
valve type (Rochester), check the spring
tension, the plastic cam, & metering rods for
6) Secondary not opening. dragging or sticking.
7) Wrong main jets or
rods.
7) Check and replace, as necessary.
8) Dragging brakes.
8) Fix brake problem.
9) Low tire pressure.
9) Increase tire pressure to factory specs.
10) Automatic
Transmission malfunction. 10) Diagnose & fix automatic transmission.
11) Wrong or
malfunctioning thermostat
in cooling system.
12) Blocked or leaking
exhaust heat passage in
intake manifold.
13) Defective accessory
(power steering pump, air
conditioning compressor,
etc.) causing drag on
engine.
14) Wheels out of
alignment.
Flooding
Gas pours out when
engine turned off.
11)
Replace thermostat.
12) Remove manifold & fix it.
13) Diagnose & repair or replace the
accessory unit.
15) Poor driving habits.
14) Have wheels professionally aligned.
15) Reduce speed, quick acceleration,
screeching around corners etc.
16) Float level much too
high, or float partly sunk.
16) Set to factory recommendations, & check
float weight, replace if necessary.
17) Ignition problems or
needs a tune up.
1) Charcoal cannister is
saturated with gas.
17) Do complete tune up & physically inspect
the cap, rotor, wires, coil, plugs, points, etc.
in addition to checking on the scope.
1) Replace the charcoal cannister.
2) Replace the defective valve or fix other
2) Vent valve is not
cause of it's malfunctioning (like loose
functioning properly.
connection, etc).
3) Kinked hoses in venting 3) Replace the kinked hoses with correct
system.
kind.
4) Stuck or blocked check
valves in the vent hoses
or elsewhere in the
system.
4) Replace the check valves.
5) Gas tank vent is
5) Fix it or replace the gas cap if vent is in
blocked.
the cap.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
6) Gas line located too
close to a heat source
(such as a radiator hose
or exhaust manifold).
Gas pours out when the
engine is running.
Note 1: often removing
the carb from the
manifold & shaking it
hard a couple of times
will effect a cure.
1) Dirt stuck on tip of the
needle & seat.
2) Idle speed is set too
fast.
CORRECTION
6) This causes the fuel to expand & be
forced past the needle & seat. Also, fuel can
boil in the carb if there are improper or
missing gaskets or spacers between the carb
& manifold. A heat riser stuck in the closed
position will also cause boiling and flooding.
1) Clean the tip off carefully or replace the
needle & seat and clean the entire fuel
system out. Check for defect cannister,
causing debris or fuel contamination.
2) Reduce the idle speed to factory specs. If
it will not idle slowly, the mixture is set too
lean, or the idle solenoid is not functioning
right, or there is a vacuum leak somewhere
(usually not in the carb).
Note 2: if procedure in
note 1 doesn't cure it, the
problems is almost
always dirt or bad gas
getting into the carb.
Carb will then need to be 3) Idle solenoid is not
disassembled to fix it.
functioning.
3) Check for power/ground up to the
solenoid. Look for dirt stuck in the solenoid
or blocking the passageways.
4) Check pressure. Use a regulator to control
4) Fuel pump pressure too the pressure or put on a new pump (don't
high.
use high pressure type).
5) Fuel pressure is
fluctuating or
"pulsing"(common on
Colts, Mitsubishi & other
Chrysler imports).
5) Control pulsing with a regulator.
6) Float is heavy, crushed 6) Replace float. Is often caused by spit-back
or sunk.
up through carb.
7) Gas is very gummy,
old, or has too much
alcohol or other chemicals 7) Clean bad gas out of carb. Clean out of
in it.
fuel system. Use only good quality gas.
8) Choke not working.
8) Check cause of choke failure, choke pulloff failure, or loss of heat to the choke.
9 Try tapping the fuel inlet or needle & seat
area with the handle of a screwdriver. If this
doesn't work, unbolt carb from manifold and
shake it hard a couple of times.
10) This is caused by the carb being
overturned or turned on its side. Remove top
and reinstall the ball.
9) Needle not seated
properly during initial fill.
10) On Mikuni carb, the
overturn ball fell out of
place
11) On carbs with
externally adjustable float 11) Excessive adjustments have caused the
levels.
seal to break. Replace the seals.
Hesitation
During
Acceleration
Engine stalls when
transmission is put into
gear.
1) Incorrect choke pull-off 1) Readjust the pull-off or replace if
adjustment.
necessary.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
2) Fast idle RPM incorrect
(too slow)
3) Engine running too lean
because of vacuum leak
or dirty jet.
Hesitation, stalling,
stumbling, flatspot, or
deadspot during
acceleration: Backfiring
or spitback up through
carb.
1) Vacuum leak.
2) Ignition timing retarded
too far.
3) Accelerator pump
nozzle has dirt in it.
4) Accelerator pump cup
swollen up from contact
with bad gas or
chemicals.
5) Economizer jet too
small or partly blocked.
6) Choke pull-off open too
far.
7) Secondary throttle
plates not closing all
the way.
CORRECTION
2) Speed up the fast idle to factory specs.
3) Check for vacuum leak. Flow test carb to
check jetting. 3) Check for vacuum leak.
1) Check for vacuum leak & fix it.
2) Reset timing.
3) Clean out the nozzle tip.
4) Replace the pump cup.
5) Clean out economizer jet & check the
size.
6) Adjust the pull-off tighter.
7) Fix it.
8) Connect up right. Be especially careful of
8) Vacuum hoses hooked the EGR & Dist. connections: sometimes the
pipe locations are reversed on Rochesters.
up wrong.
9) Idle jet partly blocked
9) Clean out the jet & any other dirt that is in
with dirt.
there.
10) Distributor timing not
advancing properly. (worn
breaker plate, worn shaft,
pin hole in diaphram,
10) Check distributor and all related systems
crack in hose, etc.)
carefully. Replace defective parts.
Hesitation, deadspot or
stalling that only seems
to occur after the first
mile of warmup. pot
1) Defective electric assist
on choke
1) Replace it.
2) Defective accelerator
pump (low output).
2) Replace it.
3) Float level setting very
low.
3) Adjust to factory specs.
4) Bad ignition condenser. 4) Replace it.
5) Inspect and clean passages in the intake
5) Plugged heat crossover manifold and heads. Test heat riser valve,
system in manifold.
replace if defective.
6) Check and replace as necessary: heat
6) Defective source of hot shroud duct, temperature sensor, vacuum
air up to the carburetor.
door motor, manifold vacuum supply.
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Carburetor Troubleshooting Guide.
Problem
CONDITION
POSSIBLE CAUSE
CORRECTION
7) Defective thermal
switch.
7) Check all thermal switches, especially
those connected with the distributor, EGR, &
vacuum motors in the air cleaner housing.
1) This will isolate bad rings, or valves, worn
cylinder walls, defective cylinder head or
gaskets and excessive carbon deposits.
2) Check at variable speeds for excessive
2) Check dwell variations. distributor shaft wear, faulty
1)Run a compression
Compression or Misfiring check.
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